(1) Field of the Invention
The present invention relates generally to an engine and more particularly to an engine in which an oil consumption and a leakage of blow-by gas can be stably reduced even after operation for a long period of time.
(2) Description of Related Art
Conventionally, there has been proposed an engine including: a first land, a first ring groove, a second land, a second ring groove, a third land, a third ring groove, and a piston skirt formed on a peripheral wall of a piston in this order from a side of a piston head; a first pressure ring fitted in the first ring groove; a second pressure ring fitted in the second ring groove; an oil ring fitted in the third ring groove; and an indentation formed at the second land (see Japanese Patent Application Laid-Open No. 10-54297 (see FIGS. 2 and 4), for example).
This type of engine has advantages that a capacity of a space facing the second land increases by a capacity of the indentation of the second land and that variation in gas pressure in the space facing the second land during upward and downward movements of the piston can be suppressed.
However, this related art has a problem, because a groove is formed in an intermediate portion of an outer peripheral face of the second land and the groove forms the indentation of the second land.
<<Problems>>
After operation for a long period of time, an oil consumption amount and a leakage of blow-by gas increase.
Because the groove is formed in the intermediate portion of the outer peripheral face of the second land and the groove forms the indentation of the second land, rigidity of the second land is significantly reduced, the piston vibrates, and the first ring groove and the second ring groove are liable to be deformed and abnormally worn.
Therefore, though the indentation formed at the second land can suppress the variation in the gas pressure in the space facing the second land during the upward and downward movements of the piston, the vibration of the piston and the deformation and the abnormal wear of the ring grooves make behavior of the first oil ring and the second oil ring unstable and the oil consumption and the leakage of the blow-by gas increase after the operation for the long period of time due to degradation of sealing performance of the first oil ring and the second oil ring.